1. Field
The present invention relates generally to a system and methods aimed at surgery, and more particularly to a system and methods for monitoring nervous tissue to assess the integrity of a pedicle wall during or after pilot hole formation and before, during, or after screw implantation, particularly in the thoracic spine.
2. Background
Pedicle fixation is one of the most common methods used to fix vertebra relative to each other in the rapidly expanding discipline of spinal fusion. Pedicle fixation generally involves the implantation of a pedicle screw in each of at least two adjacent vertebra and securing of a rod thereafter to each pedicle screw, eliminating movement between the vertebra. By way of example, FIG. 1 depicts a cross sectional view of a vertebra 1 with attached pedicle screw 7. The pedicle 2 is essentially a narrow bone bridge connecting the vertebral body 3 with the posterior elements 4 of the vertebra 1. When properly positioned the pedicle screw 7 is anchored through the center of the pedicle and into the vertebral body 3 for maximum purchase.
Because of the extreme closeness of neural tissue all around the pedicle, the risk of neurological impairment is a paramount concern during pedicle screw fixation, as it is in all procedures performed in close proximity to neural tissue. The spinal canal 5, which houses the spinal cord in the cervical and thoracic spine, and nerve roots in the lumbar spine, lies just medial to the pedicle, and exiting nerve roots leave the spinal canal 5 directly above and below the pedicle 2. There is thus little room for error when implanting pedicle screws and any such error may have dire consequences. If the pedicle (or more specifically, the cortex of the medial wall, lateral wall, superior wall and/or inferior wall) is breached, cracked, or otherwise compromised, the patient may experience neurologic deficit due to unwanted contact between the pedicle screw (or other instruments used during screw placement or pilot hole formation) and delicate neural structures such as the spinal cord or nerve roots. The neurologic deficit may range from a slight loss of sensation to paralysis. Even absent neurologic impairment, an improperly placed pedicle screw still oftentimes necessitates revision surgery, which is disadvantageously painful for the patient and costly, both in terms of recovery time and hospitalization.
It should be appreciated that the “thoracic” region, as used herein and indicated in FIG. 2, includes the thoracic vertebra as well as the first lumbar vertebra L1 (also termed the “thoracolumbar” region). The “lumbar” spine, as used herein, begins at L2 and includes the sacral vertebra (also termed “lumbosacral” region). This is to account for the fact that the spinal cord actually ends at L1 and thus cord stimulation at L1 is a factor.
To combat the risk of neurological impairment during pedicle screw fixation in the lumbar spine, many surgeons rely on neurophysiology monitoring to monitor nerves and help avoid, or at least alert them to, a pedicle breach. Pedicle integrity assessments, sometimes called screw tests, capitalize on the insulating characteristics of bone and the conductivity of the neural structures. That is, if the pedicle is breached, a pulse of electrical current applied to the pedicle screw and/or the pilot hole (prior to screw introduction) will cause the nearby nerve root to depolarize and various muscle groups innervated by the nerve will contract. If the pedicle wall has not been breached, the insulating nature of the pedicle will prevent the stimulation signal from depolarizing the given nerve roots such that the associated muscle groups will not twitch. EMG systems may be used to augment the ability to detect muscle response to the stimulus.
Screw testing may greatly reduce the risk associated with implanting a pedicle screw in the lumbar region. This is not necessarily so in the thoracic region because the presence of the spinal cord in the spinal canal (as opposed to only nerve roots in the spinal canal of the lumbar region) complicates the screw test. While a screw test in the thoracic region may reveal lateral, superior, and/or inferior pedicle breaches, which would result in stimulation of a nerve root, a medial breach may go undetected because the same stimulation signal may not produce a measurable response when communicating with the spinal cord. With the presence of the spinal cord and the smaller size of the pedicle structure in the thoracic region, the ability to accurately test pedicle integrity may be every bit as beneficial, if not more so, as it has proven to be in the lumbar region. Although not a traditional concern in cervical screw placement (cervical screws generally placed in lateral mass vs. the pedicle), pedicle screw testing my be of benefit if pedicle-based fixation in the cervical spine becomes viable based on technical advances in screw technology.
Based on the foregoing, a need exists to better assess the integrity of pedicles during pedicle fixation in the presence of the spinal cord, particularly in the thoracic region of the spine. The invention described herein is aimed at addressing this need.